System and method for recovering natural gas liquids with auto refrigeration system

ABSTRACT

The various embodiments herein provide a method and system for a process configuration with internal refrigeration for light gas and a process configuration with external refrigeration system for rich gas. The self-refrigeration unit comprising an open-closed cycle of refrigeration provides a required refrigeration load and the external refrigeration unit provides a refrigeration load corresponding to variations in a feed composition percentage. The configuration with internal refrigeration system utilizes a slip stream from or near the bottom of the demethanizer as a mixed refrigerant.

BACKGROUND

1. Technical Field

The embodiments herein generally relate to a recovery process of naturalgas liquids. The embodiments herein particularly relate to a recoveryprocess of natural gas liquids with high level of liquids recovery andmore particularly relates to a process of reducing an amount ofrefrigeration load in a recovery process of the natural gas liquids.

2. Description of the Related Art

Natural gas liquids (NGL) recovery refers to the process of removing andgathering heavier hydrocarbon products from natural gas. These heavierhydrocarbons must be separated from methane to be recovered as naturalgas liquids. These valuable natural gas liquids consist of ethane,propane, butane, and other heavier hydrocarbons. The increase in energyresources, costs and also economic issues, resulted in more complex andmore efficient recovery units for recovering the gaseous liquids. Newgeneration of NGL units have been created based on the reduction offixed and operating costs for a defined output.

The existing processes designated to update process configurations aredefined to aim at finding the new ways for maximizing the efficiency ofexisting equipments and reducing the operational costs. The major costsof the NGL recovery system are related to the refrigeration required forgas refrigerating. In the low temperature configurations, the requiredlevel of refrigeration is provided through a propane refrigerationcycle. In some cases, the mixed refrigerants and cascade refrigerationsystems are used. Besides a refrigeration cycle, some amount ofrefrigeration load is either provided by Joule-Thomson expansion orexpansion through a turbo expander. In a configuration, the requiredlevel of refrigeration depends on the integration of the processes andthe number of cool boxes. The combined heating and cooling curves of theheat exchangers indicate an efficiency of this type of equipments. Theoptimal combined curves and high efficiency of cool boxes enable toachieve more integrated configurations. The more integrated a unit is,the lesser will be the fixed and operating costs.

A gas composition highly effects an economic situation of the NGLrecovery unit and a selection of the process. The gas containing higheramounts of hydrocarbons, which can turn into liquid, will generate moreliquid products resulting in a higher efficiency in return for theapplied equipments. The richer the gas, the more is refrigeration loadand a constant investment cost and also the heat exchangers with ahigher heat transfer level are required. The light gases need morespecific process conditions to reach to the higher recovery levels. Inthe units where the gas feed is rich, the usage of an externalrefrigeration system for achieving the higher recovery levels would beinevitable. In the case of the light gases there is no need to use theexternal refrigeration systems for achieving the higher recovery levels.

There are many expansion processes, which are commonly applied for thehigh compression feeds for recovering the liquid hydrocarbons in the gasprocess industries, specifically in the recovery of ethane and propane.In many of the conventional processes of NGL expansions, the gas feedsare refrigerated near to a relatively low temperature aiming at apartial liquidation, typically through a heat exchanger with a gasstream existing out from the upper section of the distillation tower andthe lateral boilers. Some of the other existing techniques use propaneas a refrigerant for external refrigeration or the propane refrigeration(PR) in the recovery of the heavier hydrocarbons.

As an improvement over the conventional processes, a new process calledGas Sub cooled Process (GSP) was developed. This process modifiedprevalent processes in some different parts. A portion of the gas isthoroughly delivered to a converter and liquefied through the outputstream on the top of the tower. Then, this stream is sent to the topsection of the distillation tower as an input feed stream and also asthe reflux. The required work for a compression of the residue gas isless than the work, which is needed for the prevalent expanderprocesses. Typically, the amount of energy in horsepower is less thanthat of the PR process with a recovery grade of 92%.

Another novel process called Cold Residue Recycle (CRR) is the rectifiedmodel of GSP process for achieving higher recovery efficiency. Thestream path of the process is similar to that of GSP, except that thestream on the top of the tower in which a portion of residue gas ispresent, is sent back to a distillation tower as the extra returnstream. The CRR process is used in high amounts of recovery.

Another process called Recycle Split Vapor (RSV) like CRR uses a feedvapor division process for providing a portion of ethane recovery in thetower. The return stream of the methane for the tower is supplied byextracting a small portion of the residual gas, which is recompressedand liquefied. The pressure of this stream is reduced to that of anentry section, which provides a portion of the feed, required for thetop of the tower. The high pressure of this methane stream allows thegas in the top of the tower to be used for the process of liquefying andrefrigerating. Hence, a portion of vapor feed can directly be deliveredto the tower.

Yet another process called Recycle Split Vapor with Enrichment (RSVE) isthe enhancement of the RSV process. Unlike RSV process, the returnstream is extracted from the compressed residual gas and is mixed withthe vapor stream feed before getting refrigerated and liquefied. Hencethere is no need for a separate exchanger.

The development of the other NGL process based on a turbo expander leadsto the recovery of increased NGL. This process uses a portion of thestream at the bottom of the distillation tower as a mixed refrigerant.The mixed refrigerant is partially or totally evaporated to provide therequired refrigeration for refrigerating the input gas, which normallycovers the external refrigeration. The vapor generated through thisauto-refrigeration cycle is compressed again and sent back to the bottomof the tower. This vapor is used as a residue gas.

In an open-cycle refrigeration system, which aims to improve anefficiency and an economy of NGL recovery processes, a portion of astream which contains liquid hydrocarbons is depleted from the lowerparts of the distillation tower. This side stream warmed and expanded tocreate a diphase system for the separation of heavy hydrocarbon fluidsand also to provide a vapor phase is sent back to the tower. The warmand expanded vapor returned back to the tower increases theconcentration of methane and ethane. As a result of which, thetemperature profile is decreased and separation efficiency is increased.

It is found that four configurations (GSP, CRR, RSV and RSVE) are usedto recover ethane for more than 90 percents. But the feed compositionbased on which CRR, RSV, RSVE were introduced contained 92.5% methane.In other words all the prior processes worked on lighter gases. Thecited prior processes gave less emphasis on rich gases.

In the light of the foregoing discussions, there exists a need toprovide a process for recovering natural gas liquids, which givesemphasis on rich gases. There also exists a need to provide a processfor recovering the natural gas liquids to reduce the equipment cost andoperational cost and increase in recovery efficiency. Furthermore thereexists a need to provide a process for recovering the natural gasliquids and for proving an efficient refrigeration system resulting in areduction of overall cost of the process.

The abovementioned shortcomings, disadvantages and problems areaddressed herein and which will be understood by reading and studyingthe following specification.

OBJECT OF THE EMBODIMENTS

The primary object of the embodiments herein is to provide a method anda system for providing a required refrigeration load in a recovery ofnatural gas liquid so that the energy required by the system is reduceddrastically.

Another object of the embodiments herein is to provide a method and asystem for providing a required refrigeration load in a natural gasliquid recovery using an open-closed self-refrigeration system.

Yet another object of the embodiments herein is to provide a system andmethod for analyzing the configuration capability under circumstances inwhich the composition percentage is not stable and varied

Yet another object of the embodiments herein is to provide a method anda system for providing a required refrigeration load in a natural gasliquid recovery to reduce the required energy of the overall system.

Yet another object of the embodiments herein is to provide a method andsystem for providing a refrigeration load, which is proportional to thevariations of feed composition percentage.

Yet another object of the embodiments herein is to provide a method anda system for providing a required refrigeration load in a natural gasliquid recovery using least number of multi-stream converters, therebyleading to an integration of a process operation.

Yet another object of the embodiments herein is to provide a method anda system for a natural gas liquid recovery by providing a configuration,which performs on rich gases with high levels of liquid recovery.

Yet another object of the embodiments herein is to provide a method anda system for natural gas liquid recovery to reduce an energy consumptionof the system.

Yet another object of the present invention is to provide a method and asystem for natural gas liquid recovery to provide an inherent integrityresulting in reduction of numbers and volume of the applied processequipments.

Yet another object of the embodiments herein is to provide a method anda system for natural gas liquid recovery in which a distillation toweris used in the separation system thereby reducing the number ofsubsidiary systems and the refrigeration load to provide an optimalfunctioning of the configuration.

Yet another object of the embodiments herein is to provide a method anda system for natural gas liquid recovery in which the energy consumptionis less than that of similar processes and systems.

These and other objects and advantages of the embodiments herein willbecome readily apparent from the following summary and the detaileddescription taken in conjunction with the accompanying drawings.

SUMMARY

The various embodiments herein provide a method and system for a processconfiguration with internal refrigeration for light gas and a processconfiguration with external refrigeration system for rich gas. The twoprocesses are employed with a demethanizer column. The system and methodprovides a novel configuration which employs an-open closed selfrefrigeration system to provide a required refrigeration load. Thesystem and method detects for a need for an external refrigeration unitor internal refrigeration unit based on the variations in the feedcomposition percentage. The system and method further provides anexternal refrigeration cycle using propane as a refrigerant. Theconfiguration with internal refrigeration system utilizes a slip streamfrom or near the bottom of the demethanizer as a mixed refrigerant.

According to an embodiment, an internal refrigeration method forseparating a residue gas from an input gas to recover a natural liquidgas through a self-refrigeration process comprising the steps of feedingan input gas into an internal refrigeration system. The input gas iscondensed in a first multi-flow heat exchanger. An output of the firstmulti-flow heat exchanger is separated into a fluid stream and a gasstream using a first drum separator. The separated gas stream obtainedfrom the first drum separator is divided into a first gas part and asecond gas part. The separated fluid stream obtained from the first drumseparator is divided into a first fluid part and a second fluid part.The first fluid part obtained from the first drum separator is passed toa middle section of a demethanizer column through a Joule-Thomsonexpansion valve for further fractionating. The second fluid partobtained from the first drum separator is passed to a top rectifyingsection in the demethanizer column through a second multi-flow heatexchanger and a first expansion valve to form a residue gas. The firstgas part is passed to a work expander device. A gas stream output of thework expander device is passed to an area below a top rectifying sectionin the demethanizer column as an expander discharge. The second gas partis passed through the second multi-flow heat exchanger and a secondexpansion valve to the top rectifying section in the demethanizer columnto form a reflux liquid. The residue gas from the top rectifying sectionof the demethanizer column is passed through the second multi-flow heatexchanger and the first multi-flow heat exchanger to a first compressorfor further compression and wherein the first compressor compresses theresidue gas using a work extracted from the expander device. Acompressed residue gas output from the first compressor is passed to asecond compressor for further compressing the residue gas. Thecompressed residue gas is passing further through a first air cooler. Anoutput of the first cooler is forwarded as a gas refrigerant. Thevolatile components are stripped off from a liquid collected at a bottomportion of the demethanizer column by circulating the liquids from fourliquid draw trays provided in the bottom portion of the demethanizercolumn through the first multi-flow heat exchanger and the secondmulti-flow heat exchanger and collecting back the liquids at thedemethanizer column. The liquids are recovered from a bottom portion ofthe demethanizer column.

The step of stripping off the volatile components from the liquidcollected at the bottom portion of the demethanizer column comprisespassing a liquid drawn from a first liquid tray through the secondmulti-flow heat exchanger before being collected back at thedemethanizer. A liquid drawing from a second liquid tray is passedthrough the second multi-flow heat exchanger before being collected backat the demethanizer Wherein a direction of flow of the liquid from thefirst liquid tray through the second multi-flow heat exchanger and adirection of flow of the liquid from the second liquid tray through thesecond multi-flow heat exchanger are mutually opposite. A liquid drawnfrom a third liquid tray is passed through the first multi-flow heatexchanger before being collected back at the demethanizer. A liquiddrawn from a fourth liquid tray is passed through the first multi-flowheat exchanger before being collected back at the demethanizer. Whereina direction of flow of the liquid from the third liquid tray through thefirst multi-flow heat exchanger and a direction of flow of the liquidfrom the fourth liquid tray through the first multi-flow heat exchangerare mutually opposite.

The step of drawing liquid from the fourth liquid tray and passingthrough the multi-flow heat exchanger before being collected at thedemethanizer column comprises passing a liquid drawn from the fourthtray through the first multi-flow heat exchanger. An output of the firstmulti-flow heat exchanger is passed to a second drum separator through athird expansion valve. A vapor output from the second drum is fed to afirst stage of a third compressor. An output of the third compressor ispassed to a second air cooler through a third multi-flow heat exchanger.An output of the second air cooler is forwarded to a third drumseparator for dividing the output of the second air cooler into a vaporpart and a liquid part. The liquid part obtained from the second aircooler is divided into a third liquid part and a fourth liquid part. Thevapor part and the third liquid part obtained from the third drumseparator are combined and forwarded to the demethanizer. The fourthliquid part obtained from the third drum separator is passed to thethird multi-flow heat exchanger through a fourth expansion valve. Anoutput of the third multi-flow heat exchanger is divided into a fifthliquid part and a sixth liquid part. The fifth liquid part from theoutput of the third multi-flow heat exchanger is passed to the seconddrum separator. The sixth liquid part from the output of the thirdmulti-flow heat exchanger is fed to a fourth drum separator to obtain avaporized liquid and a non vaporized liquid. The vaporized liquidobtained from the fourth drum separator is forwarded to a second stageof the third compressor. The non-vaporized liquid from the fourth drumseparator and the non-vaporized liquid from the second drum separatorare combined and forwarded to the demethanizer. The input gas is a lightgas containing a low amount of hydrocarbons that are converted into anatural gas liquid. The natural gas liquid has a lower volatility degreeas compared to that of the residue gas.

According to an embodiment herein, a method of external refrigerationfor separating a residue gas from an input gas to recover a naturalliquid gas through a propane-refrigeration process comprising steps offeeding an input gas into an internal refrigeration system. The inputgas is condensed in a first multi-flow heat exchanger. An output of thefirst multi-flow heat exchanger is separated into a fluid stream and agas stream using a first drum separator. The separated gas streamobtained from the first drum separator is divided into a first gas partand a second gas part. The separated fluid stream obtained from thefirst drum separator is divided into a first fluid part and a secondfluid part. The first fluid part obtained from the first drum separatoris passed to a middle section of a demethanizer column through aJoule-Thomson expansion valve for further fractionating. The secondfluid part obtained from the first drum separator is passed to a toprectifying section in the demethanizer column through a secondmulti-flow heat exchanger and a first expansion valve to form a residuegas. The first gas part is passed to a work expander device. A gasstream output of the work expander device is passed to an area below atop rectifying section in the demethanizer column as an expanderdischarge. The second gas part is passed through the second multi-flowheat exchanger and a second expansion valve to the top rectifyingsection in the demethanizer column to form a reflux liquid. The residuegas from the top rectifying section of the demethanizer column is passedthrough the second multi-flow heat exchanger and the first multi-flowheat exchanger to a first compressor for further compression and whereinthe first compressor compresses the residue gas using a work extractedfrom the expander device. A compressed residue gas output from the firstcompressor is passed to a second compressor for further compressing theresidue gas. The compressed residue gas is passing further through afirst air cooler. An output of the first cooler is forwarded as a gasrefrigerant. The volatile components are stripped off from a liquidcollected at a bottom portion of the demethanizer column by circulatingthe liquids from four liquid draw trays provided in the bottom portionof the demethanizer column through the first multi-flow heat exchangerand the second multi-flow heat exchanger and collecting back the liquidsat the demethanizer column. The liquids are recovered from a bottomportion of the demethanizer column. An external refrigeration process isprovided using liquid propane.

The step of stripping off the volatile components from the liquidcollected at the bottom portion of the demethanizer column comprisespassing a liquid drawn from a first liquid tray through the secondmulti-flow heat exchanger before being collected back at thedemethanizer. A liquid drawing from a second liquid tray is passedthrough the second multi-flow heat exchanger before being collected backat the demethanizer Wherein a direction of flow of the liquid from thefirst liquid tray through the second multi-flow heat exchanger and adirection of flow of the liquid from the second liquid tray through thesecond multi-flow heat exchanger are mutually opposite. A liquid drawnfrom a third liquid tray is passed through the first multi-flow heatexchanger before being collected back at the demethanizer. A liquiddrawn from a fourth liquid tray is passed through the first multi-flowheat exchanger before being collected back at the demethanizer. Whereina direction of flow of the liquid from the third liquid tray through thefirst multi-flow heat exchanger and a direction of flow of the liquidfrom the fourth liquid tray through the first multi-flow heat exchangerare mutually opposite.

The step of providing external refrigeration using propane comprisingthe steps of feeding compressed liquid propane from a third compressorinto a second air cooler. An output of the second air cooler isforwarded to a second drum separator through a third expansion valve. Aliquid propane output from the second drum separator is passed through afourth expansion valve. A vapor gas output of liquid propane from thesecond drum separator forwarded to the third stage of a thirdcompressor. An output liquid propane is passed from the second drumseparator passed through the fourth expansion valve. An output liquidpropane passed through the fourth expansion valve is divided into afirst liquid propane part and a liquid propane part. The first liquidpropane part output from the fourth expansion valve is fed into a firstmulti-flow heat exchanger for providing required refrigeration forcooling the feed gas. The first liquid propane part output from thefirst multi-flow heat exchanger is forwarded to a third drum separator.The second liquid propane part output from the fourth expansion valve isfed to the third drum separator directly through a sixth expansionvalve. A vapor gas output of the third drum separator is forwarded intoa second stage of the third compressor. A non vaporized liquid outputfrom the third drum separator is fed to a fourth drum separator afterpassing the non vaporized liquid output from the third drum separatorthrough a seventh expansion valve, the second multi-flow heat exchangerand the first multi-flow heat exchanger. A vaporized output from thefourth drum separator is passed to a first stage of the thirdcompressor. A liquid propane output from the fourth drum separator isforwarded as an external refrigerant.

According to an embodiment herein, a system for separating a residue gasfrom an input gas to recover a natural liquid gas comprising a selfrefrigeration unit comprising an open-closed cycle of refrigeration andan external refrigeration unit. Wherein the self-refrigeration unitcomprising an open-closed cycle of refrigeration provides a requiredrefrigeration load and the external refrigeration unit provides arefrigeration load corresponding to variations in a feed compositionpercentage.

According to an embodiment herein, a process of internal refrigerationfor separating a residue gas from an input gas to recover a naturalliquid gas through self-refrigeration comprises steps of: feeding theinput gas into the internal refrigeration system, condensing the inputgas in a first multi-flow heat exchanger, separating a fluid stream anda gas stream by a first separator, dividing the gas stream as a firstgas part and a second gas part and the fluid stream as a first fluidpart and a second fluid part by the first separator, forming the residuegas by combining first gas part and second fluid part, forming a liquidreflux by condensing the second gas part, producing the natural gasliquid by condensing the first fluid part,

According to an embodiment herein, the step of producing the natural gasliquid by condensing the first fluid part includes: decreasingtemperature of the first fluid stream by an expansion valve, sending thefirst fluid stream to middle section of a distillation tower, increasingtemperature of stream from middle section of the distillation tower by asecond multi-flow heat exchanger, sending output stream of the expansionvalve to the lower section of the distillation tower, producing a mixedrefrigerant by combining one of the streams from a lower section of thedistillation tower with a open cycle refrigerant, increasing thetemperature of the mixed refrigerant by the first multi-flow heatexchanger by exchanging heat with the residue gas, decreasingtemperature of output stream from the first multi-flow heat exchanger bya pressure breaker valve, increasing temperature of output stream fromthe pressure breaker valve by the first multi-flow heat exchanger byexchanging heat with the residue gas, separating unvaporized liquid fromoutput gas from the first multi-flow heat exchanger by a first two phaseseparator, sending vaporized gas stream from the first two phaseseparator to first stage of a first two phase compressor, decreasingtemperature of output stream of the first two phase compressor by athird multi-flow heat exchanger, partial conversion of output streamfrom the third multi-flow heat exchanger into liquid by a first aircooler, separating liquid from output stream of the first air cooler bya second two stage separator, dividing output gas stream from the secondtwo stage separator as a first gas stream and a second a second gasstream, increasing temperature of the first gas stream by the thirdmulti-flow heat exchanger, dividing output gas stream from the thirdmulti-flow heat exchanger as a first gas portion and a second gasportion, sending the first gas portion to second stage of the first twophase compressor, dividing the second gas portion into a gas stream anda fluid stream by a third two phase separator, compressing the gasstream from the third two phase separator by the second stage of thefirst two phase compressor, producing a vaporized refrigerant bycombining the fluid stream from the third two phase separator with theliquid from the first two phase separator and producing the natural gasliquid by combining the vaporized refrigerant with the liquid from thesecond two phase separator in lower section of the distillation tower.

According to an embodiment herein, the step of forming a liquid refluxby condensing the second gas part includes: sub cooling the second gaspart by the second multi-flow heat exchanger, reducing pressure andtemperature of output of the second multi-flow heat exchanger by anexpander and sending output of the expander to top of the distillationtower. The output of the expander is the liquid reflux.

According to an embodiment herein, the step of forming the residue gasby combining first gas part and second fluid part includes: reducingpressure and temperature of first gas part and second fluid part by anexpander, sending output of the expander to the upper section of thedistillation tower, increasing the temperature of output of the firstexpander through a second multi-flow heat exchanger and first multi-flowheat exchanger by exchanging heat with the input gas, increasingtemperature and pressure of the output gas from the first multi-flowheat exchanger by compressing in a first compressor and reducing thetemperature of the output gas from the first compressor by a second aircooler to produce a residue gas, wherein the residue gas is the closedcycle refrigerant.

According to an embodiment herein, the input gas is light gas containingless amounts of hydrocarbons and is turned into natural gas liquid.

According to an embodiment herein, the natural gas liquid is havinglower volatility degree as compared to the residue gas.

According to an embodiment herein, the distillation tower is equippedwith four liquid draw trays in the lower section to provide heat to thetower for stripping volatile components off from the liquid product.

According to an embodiment herein, a process of external refrigerationfor separating a residue gas from an input gas to recover a naturalliquid gas through propane-refrigeration comprising the steps of:distributing the propane refrigerant for the first multi-flow heatexchanger, feeding the input gas into the external refrigeration system,condensing the input gas in a first multi-flow heat exchanger,separating a fluid stream and a gas stream by a first separator,dividing the gas stream as a first gas part and a second gas part andthe fluid stream as a first fluid part and a second fluid part by thefirst separator, forming the residue gas by combining first gas part andsecond fluid part, forming a liquid reflux by condensing the second gaspart, producing the natural gas liquid by condensing the first fluidpart,

According to an embodiment herein, the step of producing the natural gasliquid by condensing the first fluid part includes: decreasingtemperature of the first fluid stream by an expansion valve, sending thefirst fluid stream to middle section of a distillation tower, increasingtemperature of stream from middle section of the distillation tower by asecond multi-flow heat exchanger by exchanging heat with the propanerefrigerant, sending output stream of the expansion valve to the lowersection of the distillation tower, increasing the temperature of thestream from lower section of the distillation tower by the firstmulti-flow heat exchanger by exchanging heat with the propanerefrigerant, decreasing temperature of output stream from the firstmulti-flow heat exchanger by a pressure breaker valve, increasingtemperature of output stream from the pressure breaker valve by thefirst multi-flow heat exchanger by exchanging heat with the propanerefrigerant, separating unvaporized liquid from output gas from thefirst multi-flow heat exchanger by a first two phase separator, sendingvaporized gas stream from the first two phase separator to first stageof a first two phase compressor, decreasing temperature of output streamof the first two phase compressor by a third multi-flow heat exchangerby exchanging heat with the propane refrigerant, partial conversion ofoutput stream from the third multi-flow heat exchanger into liquid by afirst air cooler, separating liquid from output stream of the first aircooler by a second two stage separator, dividing output gas stream fromthe second two stage separator as a first gas stream and a second asecond gas stream, increasing temperature of the first gas stream by thethird multi-flow heat exchanger by exchanging heat with the propanerefrigerant, dividing output gas stream from the third multi-flow heatexchanger as a first gas portion and a second gas portion, sending thefirst gas portion to second stage of the first two phase compressor,dividing the second gas portion into a gas stream and a fluid stream bya third two phase separator, compressing the gas stream from the thirdtwo phase separator by second stage of the first two phase compressor,producing a vaporized refrigerant by combining the fluid stream from thethird two phase separator with the liquid from the first two phaseseparator and producing the natural gas liquid by combining thevaporized refrigerant with the liquid from the second two phaseseparator in lower section of the distillation tower.

According to an embodiment herein, the step of forming a liquid refluxby condensing the second gas part includes: sub cooling the second gaspart by the second multi-flow heat exchanger, reducing pressure andtemperature of output of the second multi-flow heat exchanger by anexpander and sending output of the expander to top of the distillationtower. The output of the expander is the liquid reflux.

According to an embodiment herein, the step of forming the residue gasby combining first gas part and second fluid part includes: reducingpressure and temperature of first gas part and second fluid part by anexpander, sending an output of the expander to the upper section of thedistillation tower, increasing the temperature of output of the firstexpander through a second multi-flow heat exchanger and first multi-flowheat exchanger by exchanging heat with the input gas, increasing thetemperature and the pressure of the output gas from the first multi-flowheat exchanger by compressing in a first compressor and reducing thetemperature of the output gas from the first compressor by a second aircooler to produce a residue gas, wherein the residue gas is the closedcycle refrigerant.

These and other aspects of the embodiments herein will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following descriptions, while indicatingpreferred embodiments and numerous specific details thereof, are givenby way of illustration and not of limitation. Many changes andmodifications may be made within the scope of the embodiments hereinwithout departing from the spirit thereof, and the embodiments hereininclude all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The other objects, features and advantages will occur to those skilledin the art from the following description of the preferred embodimentand the accompanying drawings in which:

FIG. 1 illustrates a block flow diagram for gas and liquid streams in aprocess configuration with the self-refrigeration system, according toan embodiment herein.

FIG. 2 illustrates a graph indicating the composite curves of a heatexchanger MSHX-1 for a process configuration with the self-refrigerationsystem, according to an embodiment herein.

FIG. 3 illustrates a graph indicating the composite curves of a heatexchanger MSHX-2 for a process configuration with the self-refrigerationsystem, according to an embodiment herein.

FIG. 4 illustrates a block flow diagram for gas and liquid streams in aprocess configuration with the external refrigeration system, accordingto an embodiment herein.

FIG. 5 illustrates a graph indicating the composite curves of the heatexchanger MSHX-1 showing the relationship between a heat flow and atemperature for a process configuration with external refrigerationsystem, according to an embodiment herein.

FIG. 6 illustrates a graph indicating the composite curves of the heatexchanger MSHX-1 showing the relationship between an inlet feed gas anda required refrigeration flow for a process configuration with externalrefrigeration system, according to an embodiment herein.

FIG. 7 illustrates a graph indicating the composite curves of the heatexchanger MSHX-1 showing the relationship between a heat flow and atemperature for a feed 1 in a process configuration withself-refrigeration system, according to an embodiment herein.

FIG. 8 illustrates a graph indicating the composite curves of the heatexchanger MSHX-2 showing the relationship between a heat flow and atemperature for a feed 1 in a process configuration withself-refrigeration system, according to an embodiment herein.

FIG. 9 illustrates a graph indicating the composite curves of the heatexchanger MSHX-1 showing the relationship between a heat flow and atemperature for a feed 6 in a process configuration withself-refrigeration system, according to an embodiment herein.

FIG. 10 illustrates a graph indicating the composite curves of the heatexchanger MSHX-2 showing the relationship between a heat flow and atemperature for a feed 6 in a process configuration withself-refrigeration system, according to an embodiment herein.

Although the specific features of the embodiments herein are shown insome drawings and not in others. This is done for convenience only aseach feature may be combined with any or all of the other features inaccordance with the embodiments herein.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description, a reference is made to theaccompanying drawings that form a part hereof, and in which the specificembodiments that may be practiced is shown by way of illustration. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the embodiments and it is to be understood thatthe logical, mechanical and other changes may be made without departingfrom the scope of the embodiments. The following detailed description istherefore not to be taken in a limiting sense.

The various embodiments herein provide a method and system for a processconfiguration with internal refrigeration for a light gas and a processconfiguration with external refrigeration system for a rich gas. The twoprocesses are employed with the separation methods using a demethanizercolumn. The configuration with internal refrigeration system utilizes aslip stream from or near the bottom of the demethanizer as a mixedrefrigerant.

According to an embodiment herein, a process of internal refrigerationfor separating a residue gas from an input gas to recover a naturalliquid gas through a self-refrigeration comprising the steps of: feedingan input gas into the internal refrigeration system, condensing theinput gas in a first multi-flow heat exchanger, separating a fluidstream and a gas stream by a first separator, dividing the gas stream asa first gas part and a second gas part and the fluid stream as a firstfluid part and a second fluid part by the first separator, forming theresidue gas by combining first gas part and second fluid part, forming aliquid reflux by condensing the second gas part, producing the naturalgas liquid by condensing the first fluid part,

According to an embodiment herein, the step of producing the natural gasliquid by condensing the first fluid part includes: decreasing atemperature of the first fluid stream by an expansion valve, sending thefirst fluid stream to a middle section of a distillation tower,increasing a temperature of a stream from a middle section of thedistillation tower by a second multi-flow heat exchanger, sending anoutput stream of the expansion valve to the lower section of thedistillation tower, producing a mixed refrigerant by combining one ofthe streams from a lower section of the distillation tower with a opencycle refrigerant, increasing the temperature of the mixed refrigerantby the first multi-flow heat exchanger by exchanging the heat with theresidue gas, decreasing a temperature of an output stream from the firstmulti-flow heat exchanger by a pressure breaker valve, increasing atemperature of an output stream from the pressure breaker valve by thefirst multi-flow heat exchanger by exchanging the heat with the residuegas, separating non-vaporized liquid from an output gas from the firstmulti-flow heat exchanger by a first two phase separator, sending avaporized gas stream from the first two phase separator to a first stageof a first two phase compressor, decreasing a temperature of an outputstream of the first two phase compressor by a third multi-flow heatexchanger, partial conversion of an output stream from the thirdmulti-flow heat exchanger into a liquid by a first air cooler,separating a liquid from an output stream of the first air cooler by asecond two stage separator, dividing an output gas stream from thesecond two stage separator as a first gas stream and a second a secondgas stream, increasing a temperature of the first gas stream by thethird multi-flow heat exchanger, dividing an output gas stream from thethird multi-flow heat exchanger as a first gas portion and a second gasportion, sending the first gas portion to a second stage of the firsttwo phase compressor, dividing the second gas portion into a gas streamand a fluid stream by a third two phase separator, compressing the gasstream from the third two phase separator by a second stage of the firsttwo phase compressor, producing a vaporized refrigerant by combining thefluid stream from the third two phase separator with the liquid from thefirst two phase separator and producing the natural gas liquid bycombining the vaporized refrigerant with the liquid from the second twophase separator in lower section of the distillation tower.

According to an embodiment herein, the step of forming a liquid refluxby condensing the second gas part includes: sub cooling the second gaspart by the second multi-flow heat exchanger, reducing a pressure and atemperature of an output of the second multi-flow heat exchanger by anexpander and sending an output of the expander to a top portion of thedistillation tower. The output of the expander is a reflux liquid.

According to an embodiment herein, the step of forming the residue gasby combining a first gas part and a second fluid part includes: reducinga pressure and a temperature of a first gas part and a second fluid partby an expander, sending an output of the expander to the upper sectionof the distillation tower, increasing the temperature of output of thefirst expander through a second multi-flow heat exchanger and firstmulti-flow heat exchanger by exchanging heat with the input gas,increasing a temperature and a pressure of the output gas from the firstmulti-flow heat exchanger by compressing in a first compressor andreducing the temperature of the output gas from the first compressor bya second air cooler to produce a residue gas, wherein the residue gas isthe closed cycle refrigerant.

According to an embodiment herein, the input gas is light gas containingless amounts of hydrocarbons that are turned into natural gas liquid.

According to an embodiment herein, the natural gas liquid is havinglower volatility degree as compared to the residue gas.

According to an embodiment herein, the distillation tower is equippedwith four liquid draw trays in a lower section to provide a heat to thetower for stripping off the volatile components from the liquid product.

According to an embodiment herein, a process of external refrigerationfor separating a residue gas from an input gas to recover a naturalliquid gas through a propane-refrigeration comprising the steps of:distributing the propane refrigerant for the first multi-flow heatexchanger, feeding the input gas into the external refrigeration system,condensing the input gas in a first multi-flow heat exchanger,separating a fluid stream and a gas stream by a first separator,dividing the gas stream as a first gas part and a second gas part andthe fluid stream as a first fluid part and a second fluid part by thefirst separator, forming the residue gas by combining first gas part andsecond fluid part, forming a liquid reflux by condensing the second gaspart, producing the natural gas liquid by condensing the first fluidpart,

According to an embodiment herein, the step of producing the natural gasliquid by condensing the first fluid part includes: decreasing atemperature of the first fluid stream by an expansion valve, sending thefirst fluid stream to a middle section of a distillation tower,increasing a temperature of a stream from the middle section of thedistillation tower by a second multi-flow heat exchanger by exchangingthe heat with the propane refrigerant, sending an output stream of theexpansion valve to the lower section of the distillation tower,increasing the temperature of the stream from a lower section of thedistillation tower by the first multi-flow heat exchanger by exchangingthe heat with the propane refrigerant, decreasing a temperature of anoutput stream from the first multi-flow heat exchanger by a pressurebreaker valve, increasing a temperature of an output stream from thepressure breaker valve by the first multi-flow heat exchanger byexchanging the heat with the propane refrigerant, separatingnon-vaporized liquid from an output gas from the first multi-flow heatexchanger by a first two phase separator, sending a vaporized gas streamfrom the first two phase separator to a first stage of a first two phasecompressor, decreasing a temperature of an output stream of the firsttwo phase compressor by a third multi-flow heat exchanger by exchangingthe heat with the propane refrigerant, partial conversion of an outputstream from the third multi-flow heat exchanger into a liquid by a firstair cooler, separating a liquid from an output stream of the first aircooler by a second two stage separator, dividing an output gas streamfrom the second two stage separator as a first gas stream and a second asecond gas stream, increasing a temperature of the first gas stream bythe third multi-flow heat exchanger by exchanging the heat with thepropane refrigerant, dividing an output gas stream from the thirdmulti-flow heat exchanger as a first gas portion and a second gasportion, sending the first gas portion to a second stage of the firsttwo phase compressor, dividing the second gas portion into a gas streamand a fluid stream by a third two phase separator, compressing the gasstream from the third two phase separator by a second stage of the firsttwo phase compressor, producing a vaporized refrigerant by combining thefluid stream from the third two phase separator with the liquid from thefirst two phase separator and producing the natural gas liquid bycombining the vaporized refrigerant with the liquid from the second twophase separator in lower section of the distillation tower.

According to an embodiment herein, the step of forming a liquid refluxby condensing the second gas part includes: sub cooling the second gaspart by the second multi-flow heat exchanger, reducing a pressure and atemperature of an output of the second multi-flow heat exchanger by anexpander and sending an output of the expander to a top of thedistillation tower. The output of the expander is the liquid reflux.

According to an embodiment herein, the step of forming the residue gasby combining a first gas part and a second fluid part includes: reducinga pressure and a temperature of the first gas part and a second fluidpart by an expander, sending an output of the expander to the uppersection of the distillation tower, increasing the temperature of anoutput of the first expander through a second multi-flow heat exchangerand a first multi-flow heat exchanger by exchanging the heat with theinput gas, increasing a temperature and a pressure of the output gasfrom the first multi-flow heat exchanger by compressing in a firstcompressor and reducing the temperature of the output gas from the firstcompressor by a second air cooler to produce a residue gas, wherein theresidue gas is the closed cycle refrigerant.

FIG. 1 is a stream diagram of a process configuration with the internalrefrigeration system, according to an embodiment herein. With respect toFIG. 1, a feed gas comprising a pretreated and clean natural gas orrefinery gas stream is introduced into the illustrated process throughan inlet stream Feed Gas at a temperature of about 37.7° C. and anelevated pressure of about 63 bars. The stream is cooled in the multistream heat exchanger MSHX-1 to reduce the temperature of the stream toabout −32.5° C. The output stream 1 from MSHX-1 is passed to the flashdrum D-1 for a separation of the condensed liquid, if any. A portion ofthe liquid 4 is introduced into the middle of demethanizer column forfurther fractionation. A J-T valve decreases its temperature to about−52° C. before entering to the tower. Another portion, stream 3, isexpanded through the expansion valve VLV-3 and fed to the demethanizer.The outlet vapor stream 2 from the drum D-1 is divided into twoportions, the main portion 2 and the remaining portion 6. The mainportion 2 which is about 60%, is expanded through a work-expansionturbine Ex-1 prior to entering the demethanizer right below the toprectifying section as an expander discharge 8. The remaining vaporportion 6 is cooled to substantially condensation, and in most casessub-cooling, to approximately −95° C. via MSHX-2. This sub-cooled liquidstream 9 is expanded through the expansion valve VLV-2 and fed to top ofthe demethanizer as reflux liquid.

The demethanizer operated at approximately 25 bars is a distillationcolumn containing conventional kinds of trays applied in thedemethanizer towers. The demethanizer is equipped with four liquid drawtrays in the lower section of the column to provide a heat to the columnfor stripping off the volatile components from the bottom liquidproduct. This is accomplished via the use of two multi stream heatexchangers MSHX-1, MSHX-2. The liquids drawn from the side of thedemethanizer 14& 16, and 18& 20 are passed to the MSHX-2 and MSHX-1 at−54, −53, −36 & −16.5° C. respectively, and exit as streams 15&17, and19 & 37 at approximately −42, −41, −24 & 25° C., respectively, prior toreturning to the demethanizer

The residue gas 7 exiting the upper portion of the demethanizer is fedto the MSHX-2 exchanger, providing refrigeration for condensing/subcooling the vapor split stream 6 and sub cooling the liquid stream 3from the drum D-1. The residue gas 10 exiting the MSHX-2 is furtherwarmed to near the feed gas temperature via MSHX-1. The warmed residuegas 11 leaving the MSHX-1 at approximately 34° C. is sent to the suctionof the expander compressor C-1, where it is compressed to 29 bars byutilizing a work extracted from the expander Ex-1. Depending upon theneeded delivery pressure, a residue gas compressor C-2 may be needed tofurther compress the residue gas stream 12 from the compressor C-1,followed by an air cooler AC-1, prior to its final delivery at 62 bar.

In this configuration, the refrigeration provided by the residue gasfrom the demethanizer, turbo expander Ex-1 and the side liquid draws isnot sufficient to achieve a high level of ethane recovery. So, aself-refrigeration system is applied. Stream 20, the open cyclerefrigerant, is withdrawn from the chimney tray of the demethanizercolumn; the resulting mixed refrigerant is preferentially fed to theMSHX-1 for sub cooling prior to being expanded through the expansiondevice VLV-1 at 9 bar. The expanded stream 21 is directed back to theMSHX-1 providing an indirect heat exchange with the Feed gas stream andthereafter fed to the suction knockout drum D-2 where un vaporizedliquid 25, if any, is separated while the refrigerant is used to coolthe inlet gas stream. The vapor stream 24 produced in the knockout drumD-2 is withdrawn from the top thereof and fed to a first stage of atwo-stage recycle compressor C-3. The re-pressurized gas stream 28exiting compressor C-3 is cooled in MSHX-3 to 89° C., and then theoutput 29 of the heat exchanger MSHX-3 is fed to the air cooler AC-2resulting in a partial condensation. The partially condensed product 30exiting the cooler AC-2 is introduced into separator D-3 where condensedliquid is separated. A portion of the output liquid stream 34 withdrawnfrom separator D-3 (representing closed cycle refrigerant) is used as arefrigerant in the heat exchanger MSHX-3. In fact this portion providesa part of the required refrigeration in the condensation section ofopen-cycle refrigeration system. The flow rate of Stream 34, pressuredrop in the VLV-4 expansion device and the temperature of stream 35 areparameters, which should be adjusted, based on the open refrigerationcycle performance. Also these three parameters affect the air coolerperformance and its design condition. Consequently a tradeoff betweenthe compressors shaft work and condensation costs (fixed and operatingcosts related to the air cooler) will determine their optimum value.

A portion of the outlet stream 35 from MSHX-3 flows to D-4 knockout drumas the closed cycle separator. The vapor product 27 of D-4 is introducedinto the second stage of the compressor C-3. After an indirect heatexchange with one or more process streams, the heated open and closedrefrigerant is preferably combined for simplicity and introduced intosuction knockout drum D-2 where the vaporized refrigerant is separated.The vapor stream 24 is then introduced to the first stage recyclecompressor C-3. The pressure of a liquid product drawn from D-4 isincreased by the P-2 pump, next this stream, 36 is mixed with stream 26(Separator D-2 liquid product). Finally stream 37 is introduced to thebottom of the demethanizer column (stream 37). The pressure, temperatureand flow rate of the process streams are presented in Table 1.

TABLE 1 Material balance for the processes illustrated in the FIGS. 1FIG. 1 Temperature Pressure Flow rate Stream Number (° C.) (bar)(kmole/Hr) Feed Gas 37.78 63.09 14942.28 1 −30.00 63.09 14942.28 2 — —4126.94 3 — — 1330.22 4 — — 5116.24 5 −63.25 26.80 1330.22 6 −30.0063.09 5699.11 7 −93.87 25.86 11349.28 8 −66.21 26.00 4126.94 9 −94.7026.50 5699.11 10 −36.41 25.86 11349.28 11 35.00 25.86 11349.28 12 43.8328.37 11349.28 13 62.00 11349.28 185577.88 14 −54.87 26.14 2000.15 15−42.00 26.14 1999.68 16 −51.11 26.53 2199.88 17 −40.00 26.53 2199.88 18−20.94 26.75 3549.97 19 −3.00 26.75 3550.19 20 2.42 26.82 3580.29 21−33.42 9.00 3580.29 22 35.73 9.00 3580.29 23 6.77 9.00 450.00 24 30.619.00 3635.76 25 30.61 9.00 394.54 26 31.97 26.89 394.54 27 20.00 14.0014.32 28 94.20 26.89 3650.10 29 91.42 26.89 3650.10 30 35.00 26.893650.10 31 35.00 26.89 2006.96 32 35.00 26.89 1150.20 33 34 14.70 14.00492.94 35 20.00 14.00 492.94 36 21.28 26.89 28.62 37 39.29 26.89 3580.3238 Pipeline gas 64.51 62.00 11349.28 Liquid Product 32.84 26.89 3591.57

FIG. 2 illustrates a graph indicating the composite curves of a heatexchanger MSHX-1 for a process configuration with the self-refrigerationsystem, according to an embodiment herein while FIG. 3 illustrates agraph indicating the composite curves of a heat exchanger MSHX-2 for aprocess configuration with the self-refrigeration system, according toan embodiment herein. In this configuration, the feed will get cooledoff through MSHX-1 and MSHX-2 heat exchangers, in two stages. In otherwords, two heat exchangers are considered for two types of process coldstreams: Firstly, those cold streams whose temperature is extremely low(the streams which exit from the upper section of the tower) and coolstreams whose temperature is relatively higher (the streams which exitfrom the lower sections of the tower). This method enjoys twoadvantages: firstly, it causes the process cool streams andrefrigeration cycles to optimally be recovered. And secondly, it causesthe combined curves to be in their most optimal position, which is anevidence of optimum functioning of multi-flow heat exchangers. It isworth noting that setting the operating conditions is highly importantin the unit, because they are able to strongly influence the function ofmulti-flow heat exchangers while the increase in the number of heatexchangers leads to larger optimization problem. Although aconfiguration with a multi-flow heat exchanger can be designed, but atemperature control of cold streams will be limited to achieve theoptimum point of efficiency.

The form of combined curves is an important factor illustrating theefficiency of multi-flow heat exchangers. Pinch point in this curveillustrates the extreme proximity of the curves. The structuralcapabilities of the heat exchanger determine the lowest allowedtemperature difference between cool and hot curves. The lowesttemperature difference in this curve is considered to be 2° C.

MSXH-1 exchanger decreases the temperature of the feed to 300 C. FIG. 2illustrates the combined curve of this apparatus. The requiredrefrigeration in this apparatus is provided through two refrigerationresources (open cycle and side streams of distillation tower). Thisapparatus applies warmer cool fluid process streams, and prepares feedstream for entering the MSHX-2 exchanger. MSHX-2 uses the stream number7 as the coolest process stream. FIG. 3, illustrates the combined curveof this apparatus.

FIG. 4 is a stream diagram of a process configuration with the externalrefrigeration system, according to an embodiment herein. With respect toFIG. 4, a feed gas comprising a pretreated and clean natural gas orrefinery gas stream is introduced into the illustrated process throughan inlet stream. The feed Gas is at a temperature of about 37.7° C. andan elevated pressure of about 63 bars. This stream is cooled in themulti stream heat exchanger MSHX-1 to reduce the temperature of thestream to about −29° C. The output stream 1 from MSHX-1 follows to theflash drum D-1 for separation of the condensed liquid, if any. A portionof the liquid 4 is introduced into the middle of demethanizer column forfurther fractionation. A J-T valve decreases its temperature to about−47.65° C. before entering to the tower. Another portion, stream 3, isexpanded through the expansion valve VLV-3 and fed to the demethanizer

The outlet vapor stream 2 from D-1 drum is divided into two portions,the main portion 2 and the remaining portion 6. The main portion 2 whichis about 42%, is expanded through a work-expansion turbine EX-1 prior toentering the demethanizer right below the top rectifying section asexpander discharge 8. The remaining vapor portion 6 is cooled tosubstantially condensation, and in most cases sub-cooling, toapproximately −71.5° C. via MSHX-2. This sub-cooled liquid stream 9 isexpanded through the expansion valve VLV-2 to top of the demethanizer asa reflux liquid.

The demethanizer operated at approximately 23 bar is a distillationcolumn containing conventional kinds of trays applied in thedemethanizer towers. It is equipped with four liquid draw trays in thelower section of the column to provide heat to the column for strippingoff volatile components from the bottom liquid product. This isaccomplished via the use of two multi stream heat exchangers MSHX-1,MSHX-2. The side draw liquids 14&16 and 18&20 to enter the MSHX-2 andMSHX-1 at −55.29, −51.88, −31.34 & −7.70° C. respectively, and exit asstreams 15, 17, 19 & 21 at approximately −43, −38, −20 & 35° C.,respectively, prior to returning to the demethanizer

The residue gas 7 exiting the upper portion of the demethanizer is fedto the MSHX-2 exchanger, providing refrigeration forcondensing/sub-cooling the vapor split stream 6 and sub-cooling theliquid stream 3 from the drum D-1. The residue gas 10 exiting the MSHX-2is further warmed to near the feed gas temperature via MSHX-1. Thewarmed residue gas 11 leaving the MSHX-1 at approximately 10° C. is sentto the suction of the expander compressor C-1, where it is compressed to26.6 bar by utilizing a work extracted from the expander Ex-1. Dependingupon the delivery pressure needed, a residue gas compressor C-2 may beneeded to further compress the residue gas stream 12 followed by an aircooler AC-1, prior to its final delivery at 62 bar.

In this configuration the refrigeration provided by the residue gas fromthe demethanizer, turbo expander EX-1 and side liquid draws is notsufficient to achieve high levels of ethane recovery. So, a three-stagepropane refrigeration cycle is applied in order to compare theperformance of the process with the configuration explained in thesection 2.1. It should be noted that based on the potential of theproposed configuration, distribution of the propane refrigerant in theevaporators (MSHX-1 and MSHX-2) was done in the manner that the maximumcold recovery is gained. Also integrity of the process was consideredthough using the external refrigeration system.

The outlet stream 37 from the air cooler AC-2 (Condenser of the cycle)in the temperature of 37.7° C. and pressure of 13 bar follows to VLV-4and its pressure is decreased to 6.5 bar. Stream 38 output from theVLV-4 is sent to the flash drum D-2. Gas product of the D-2 enters thethird stage of a three-stage compressor C-3. The outlet liquid propane24 from D-2 is split into two parts after passing an expansion deviceVLV-5. A portion 26 of it, 85% follows to MSHX-1 in and provides therequired refrigeration for cooling the feed gas. Outlet stream 35 fromMSHX-1 enters flash drum D-4 at −18° C. and 2.61 bars. Another portion,28 follows to D-4 after passing expansion valve VLV-7. The gas product33 of D-4 enters the second stage of compressor C-3 and the liquidproduct 29 which is the first stage refrigerant of the cycle follows tothe MSHX-2 through the expansion valve VLV-6 at −38.55° C. and 1.18bars. The outlet stream, 31 is not vaporized totally in this heatexchanger MSHX-2 so it is sent to the MSHX-1 as second evaporator.Finally the vaporized refrigerant 32 with minimum pressure enters theD-3 before following to the 1st stage of compressor C-3 at −15.65° C.and 1.17 bar. The pressure, temperature and flow rate of the processstreams are presented in Table 2.

TABLE 2 Material balance for the processes illustrated in the FIG. 4FIG. 4 Temperature Pressure Flow rate Stream Number (° C.) (bar)(kmole/Hr) Feed Gas 37.78 63.09 14942.28 1 −29.00 63.09 — 2 29.00 63.094181.66 3 −29.00 63.09 1296.35 4 — — 3689.61 5 −73.52 25.80 1296.35 6−29.00 63.09 5774.67 7 −95.50 23.86 11351.04 8 −66.53 25.00 4181.66 9−94.97 — 5774.67 10 −45.00 23.86 11351.04 11 10.00 23.86 — 12 19.7926.60 11348.39 13 101.87 62.00 — 14 −55.29 23.87 2200.00 15 −43.00 — —16 −51.88 23.88 2000.00 17 −38.00 — — 18 −31.34 23.89 3200.02 19 −20.00— — 20 −7.70 23.89 — 21 35.00 — — 22 23 10.86 6.50 672.26 24 10.86 6.502527.74 25 −17.31 2.68 — 26 −17.31 2.68 2172.08 27 −17.84 2.63 355.65 28−17.31 2.68 355.65 29 −18.01 2.61 858.64 30 −38.55 1.18 — 31 −38.52 1.18858.64 32 −15.65 1.17 — 33 −18.01 2.61 1669.10 34 −15.65 1.17 858.64 35−18.00 2.61 2172.08 36 65.29 13.00 3200.00 37 37.75 13.00 — 38 10.866.50 — Pipeline gas 64.62 62.00 11348.39 Liquid Product 25.64 23.893591.07

FIG. 5 illustrates a graph indicating the composite curves of the heatexchanger MSHX-1 showing the relationship between a heat flow and atemperature for a process configuration with external refrigerationsystem, according to an embodiment herein. The form of combined curvesis an important factor illustrating the efficiency of multi-flow heatexchangers. Pinch point in this curve illustrates the extreme proximityof the curves. The structural capabilities of the heat exchangerdetermine the lowest allowed temperature difference between cool and hotcurves. A comparison between FIGS. 3&5 shows that the multi stream heatexchangers in the FIG. 1 process are more efficient than the ones, whichused in the process with external refrigeration system (FIG. 4).

FIG. 6 illustrates a graph indicating the composite curves of the heatexchanger MSHX-1 showing the relationship between an inlet feed gas anda required refrigeration flow for a process configuration with externalrefrigeration system, according to an embodiment herein. With respect toFIG. 6, the required power increases linearly. Also the power requiredfor increasing the pressure of the residue gas and also the power ofAC-1 fan air cooler decreases, as the feed becomes richer, because thericher is the gas, the more refrigeration it will need and thereupon,the air stream discharge should be increased in air coolers. The powerof needed fan in external refrigeration system will be more due to thedifference in refrigerator discharge in internal and external systems.Mutually, the power of required fan will decrease as the gas becomesricher, but the temperature of stream entering AC-1 in FIG. 1 will behigher than that of FIG. 4, so more air stream is needed in thisprocess. In all the cases, ethane recovery was considered constant andequal in both of the configurations. So the least proximity temperature,in MSHX-1 and MSHX-2 will be 2 C. According to tables 4 and 5, as thefraction Deuterium increases in the freed, ethane recovery level willdecrease. In the mixtures which have high amounts of methane (more than80 percent), the process function will be desirable, and higher levelsof ethane recovery can be achieved as the compressor of theauto-refrigeration systems consumes lower power. And the power ofresidue gas compressor increases due to the increase of methane amount.In the mixtures containing high amounts of C²⁺ and hydrogen, thesystem's required by auto-refrigeration system (FIG. 1) increases by thedecrease in methane fraction, and the required power for residue gascompressor decreases. Though, the level of ethane and methane recoverycan be controlled in amount of +90. Although the least proximitytemperature in the [2, 2.5] range was made fixed, but the form ofcombined curves changes depending on the combination of the feed.

Tables 4, 5 and 6, indicate the required refrigeration (whether externalor internal) for feeds under investigation.

TABLE 4 Overall performance of the configuration Prior Art(s) Presentinvention % Ethane recovery 90.1 91.41 % Propane recovery 99.4 99.4Self- refrigeration compression, kW 4113 3475.36 Propane refrigeration,kW 0 0 Residue gas compression, kW 8904 9042

TABLE 5 Overall performance of the process for the different feedcompositions (FIG. 1) Feed 1 Feed 2 Feed 3 Feed 4 Feed 5 Feed 6 % Ethane94.00 93.91 93.36 91.41 92.41 90.34 recovery % Propane 99.99 99.95 99.4899.40 99.87 99.60 recovery % Methane 0.99 99.13 99.12 98.95 98.55 98.17recovery MSHEX1 2.19 2.00 2.00 2.11 2.00 2.17 minimum temperatureapproach ° C. MSHEX2 2.00 2.00 2.00 2.00 2.00 1.97 minimum temperatureapproach ° C. Self- 2272.57 2664.42 3095.82 3475.36 4105.62 4385.56refrigeration compression, kW Residue gas 9784.74 9334.59 9213.969042.51 8608.91 8321.15 compression, kW Duty of −6271.61 −8201.20−8803.4394 −7874.10 −11656.53 −12315.83 refrigeration system air cooler,kW Duty of −8836.40 −8507.09 −8232.67 −9956.04 −7464.29 −7134.98 residuegas air cooler, kW Heat flow of −334912.74 −339307.05 −341035.43−345611.94 −352546.70 −358046.23 Feed stream, kW Heat flow of −81909.12−97429.99 −104636.88 −118508.52 −140981.39 −156654.16 Liquid Product, kWHeat flow of −256054.33 −246586.35 −241124.89 −232415.69 −217971.60−208136.17 Gas Product, kW Fan power in 79.86 76.88 74.40 71.92 67.4664.48 the residue gas air cooler, kW Fan power in 55.06 67.46 79.3789.98 104.17 111.11 the air cooler of refrigeration system, kW Overall12192.22 12143.35 12463.54 12679.78 12886.16 12882.30 required power, kW

TABLE 6 Overall performance of the process for the different feedcompositions (FIG. 4) Feed 1 Feed 2 Feed 3 Feed 4 Feed 5 Feed 6 % Ethanerecovery 94 93.89 93.52 91.6 92.5 90.15 % Propane recovery 99.5 99.4599.5 99.38 99.7 99.55 % Methane recovery 99.2 98.95 99 99 97.91 97.05MSHEX1 minimum 2.5 2.77 2.77 2.77 2.77 2.77 temperature approach ° C.MSHEX2 minimum 2.14 1.95 2.71 3 2.28 2.16 temperature approach ° C.Propane refrigeration, 2972.356 3839.229 4152.029 4272.29 4852.7525078.086 kW Residue gas 9783.394 9442.384 9314.823 9020.75 8763.6658591.18 compression, kW Duty of refrigeration −9946.377 −12659.62−14348.12 −14558 −16476.32 −16974.5 system air cooler, kW Duty ofresidue gas air −6476.368 −6001.163 −5323.794 −4946.4 −5434.938−5927.524 cooler, kW Heat flow of Feed −334912.7 −339307.1 −341035.4−345612 −352546.7 −358046.2 stream, kW Heat flow of Liquid −82823.76−98315.92 −106064.6 −119402 −144425.5 −161563.7 Product, kW Heat flow ofGas −255756 −246370.3 −241175.9 −232421 −216416 −205715.3 Product, kWFan power in the residue 58.53175 49.60317 48.11508 46.131 49.6031753.57143 gas air cooler, kW Fan power in the air 94.24603 114.4144126.5476 131.071 148.9087 177.5849 cooler of refrigeration system, kWOverall required power, 12908.53 13445.63 13641.51 13470.2 13814.9313900.42 kW

As it was expected, the heavier is the gas, the more refrigeration doesit need. Also, changes in specifications of the process as thecombination of the feed varies can be followed through these two tables.Therefore the types of configurations can be compared accurately(according to FIGS. 1 and 4). As it was said previously, the process inFIG. 4 was designed, so a precise comparison can be done between thefunction of internal and external refrigeration system for the proposedconfiguration. According to operating conditions (temperature, pressureand discharge) it can be assumed that the size of separation equipmentsare the same in both of the configurations. And also MSHX-2 and MSHX-1exchangers in FIG. 4, have one more path comparing to FIG. 1.Consequently it can be said that the constant investment costs for thesetwo equipments are equal. AC-2 air coolers are in the same size in bothconfigurations (UA is the same in both of the configurations). C-3 is atwo-stage compressor in FIG. 1, whilst in FIG. 4 it is a three-stagecompressor. There are three (D-2, D-3, D-4) in both of theconfigurations, but they are bigger in FIG. 4 than in FIG. 1. So thenumber of equipments is equal in both external and internalrefrigeration systems but their size is smaller in internal system.Considering what mentioned above, in circumstances in which the feedcombination changes, operating costs can be a determining factor incomparison between the function of FIGS. 1 and 4.

FIG. 7 illustrates a graph indicating the composite curves of the heatexchanger MSHX-1 showing the relationship between a heat flow and atemperature for a feed 1 in a process configuration withself-refrigeration system, according to an embodiment herein. The formof combined curves is an important factor illustrating the efficiency ofmulti-flow heat exchangers. Pinch point in this curve illustrates theextreme proximity of the curves. The structural capabilities of the heatexchanger determine the lowest allowed temperature difference betweencool and hot curves

FIG. 8 illustrates a graph indicating the composite curves of the heatexchanger MSHX-2 showing the relationship between a heat flow and atemperature for a feed 1 in a process configuration withself-refrigeration system, according to an embodiment herein. The formof combined curves is an important factor illustrating the efficiency ofmulti-flow heat exchangers. Pinch point in this curve illustrates theextreme proximity of the curves. The structural capabilities of the heatexchanger determine the lowest allowed temperature difference betweencool and hot curves

FIG. 9 illustrates a graph indicating the composite curves of the heatexchanger MSHX-1 showing the relationship between a heat flow and atemperature for a feed 6 in a process configuration withself-refrigeration system, according to an embodiment herein. The formof combined curves is an important factor illustrating the efficiency ofmulti-flow heat exchangers. Pinch point in this curve illustrates theextreme proximity of the curves. The structural capabilities of the heatexchanger determine the lowest allowed temperature difference betweencool and hot curves

FIG. 10 illustrates a graph indicating the composite curves of the heatexchanger MSHX-2 showing the relationship between a heat flow and atemperature for a feed 6 in a process configuration withself-refrigeration system, according to an embodiment herein. The formof combined curves is an important factor illustrating the efficiency ofmulti-flow heat exchangers. Pinch point in this curve illustrates theextreme proximity of the curves. The structural capabilities of the heatexchanger determine the lowest allowed temperature difference betweencool and hot curves

The embodiments herein provide a method and a system for providingrequired refrigeration load in a natural gas liquid recovery. The methodand system provides required refrigeration load in a natural gas liquidrecovery using an open-closed self-refrigeration system. The system andmethod for analyzing the configuration capability under circumstances inwhich the composition percentage is not stable and varies is provided.The process configuration provides required refrigeration load in anatural gas liquid recovery to reduce the required energy of the overallsystem. The method and system investigates the need of the unit torefrigeration proportional to the variations of feed compositionpercentage. The process configuration uses least number of multi-streamconverters, further leading to the integration of the process operation.The process configuration here in performs on rich gases with highlevels of liquid recovery. The presented process configuration reducesover all consumed energy of the system. The method and system fornatural gas liquid recovery provides inherent integrity resulting inreduction of numbers and volume of the applied process equipments.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the embodiments herein that others can, byapplying current knowledge, readily modify and/or adapt for variousapplications such specific embodiments without departing from thegeneric concept, and, therefore, such adaptations and modificationsshould and are intended to be comprehended within the meaning and rangeof equivalents of the disclosed embodiments. It is to be understood thatthe phraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Therefore, while the embodimentsherein have been described in terms of preferred embodiments, thoseskilled in the art will recognize that the embodiments herein can bepracticed with modification within the spirit and scope of the appendedclaims.

Although the embodiments herein are described with various specificembodiments, it will be obvious for a person skilled in the art topractice the embodiments herein with modifications. However, all suchmodifications are deemed to be within the scope of the claims.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the embodimentsdescribed herein and all the statements of the scope of the embodiments,which as a matter of language might be said to fall there between.

What is claimed is:
 1. An internal refrigeration method for separating aresidue gas from an input gas to recover a natural liquid gas through aself-refrigeration process comprising steps of: feeding an input gasinto an internal refrigeration system; condensing the input gas in afirst multi-flow heat exchanger; separating an output of the firstmulti-flow heat exchanger into a fluid stream and a gas stream using afirst drum separator; dividing the separated gas stream obtained fromthe first drum separator into a first gas part and a second gas part;dividing the separated fluid stream obtained from the first drumseparator into a first fluid part and a second fluid part; passing thefirst fluid part obtained from the first drum separator to a middlesection of a demethanizer column through a Joule-Thomson expansion valvefor further fractionating; passing the second fluid part obtained fromthe first drum separator to a top rectifying section in the demethanizercolumn through a second multi-flow heat exchanger and a first expansionvalve to form a residue gas; passing the first gas part to a workexpander device; passing a gas stream output of the work expander deviceto an area below a top rectifying section in the demethanizer column asan expander discharge; passing the second gas part through the secondmulti-flow heat exchanger and a second expansion valve to the toprectifying section in the demethanizer column to form a reflux liquid;passing the residue gas from the top rectifying section of thedemethanizer column through the second multi-flow heat exchanger and thefirst multi-flow heat exchanger to a first compressor for furthercompression, and wherein the first compressor compresses the residue gasusing a work extracted from the expander device; passing a compressedresidue gas output from the first compressor to a second compressor forfurther compressing the residue gas; passing the further compressedresidue gas through a first air cooler; forwarding an output of thefirst cooler as a gas refrigerant; stripping off volatile componentsfrom a liquid collected at a bottom portion of the demethanizer columnby circulating liquids from four liquid draw trays provided in thebottom portion of the demethanizer column through the first multi-flowheat exchanger and the second multi-flow heat exchanger and collectingback the liquids at the demethanizer column; and recovering the liquidsfrom a bottom portion of the demethanizer column.
 2. The methodaccording to claim 1, wherein a step of stripping off volatilecomponents from the liquid collected at the bottom portion of thedemethanizer column comprises: drawing a liquid from a first liquid trayand passing through the second multi-flow heat exchanger before beingcollected back at the demethanizer; drawing a liquid from a secondliquid tray and passing through the second multi-flow heat exchangerbefore being collected back at the demethanizer, wherein a direction offlow of the liquid from the first liquid tray through the secondmulti-flow heat exchanger and a direction of flow of the liquid from thesecond liquid tray through the second multi-flow heat exchanger aremutually opposite; drawing a liquid from a third liquid tray and passingthrough the first multi-flow heat exchanger before being collected backat the demethanizer; drawing a liquid from a fourth liquid tray andpassing through the first multi-flow heat exchanger before beingcollected back at the demethanizer, wherein a direction of flow of theliquid from the third liquid tray through the first multi-flow heatexchanger and a direction of flow of the liquid from the fourth liquidtray through the first multi-flow heat exchanger are mutually opposite.3. The method according to claim 2, wherein step of drawing liquid fromthe fourth liquid tray and passing through the multi-flow heat exchangerbefore being collected at the demethanizer column comprises: passing aliquid drawn from the fourth tray through the first multi-flow heatexchanger; passing an output of the first multi-flow heat exchanger to asecond drum separator through a third expansion valve; feeding a vaporoutput from the second drum to a first stage of a third compressor;passing an output of the third compressor to a second air cooler througha third multi-flow heat exchanger; forwarding an output of the secondair cooler to a third drum separator for dividing the output of thesecond air cooler into a vapor part and a liquid part; dividing theliquid part obtained from the second air cooler into a third liquid partand a fourth liquid part; combining the vapor part and the third liquidpart obtained from the third drum separator; forwarding the combined thevapor part and the third liquid part obtained from the third drumseparator to the demethanizer; passing the fourth liquid part obtainedfrom the third drum separator to the third multi-flow heat exchangerthrough a fourth expansion valve; dividing an output of the thirdmulti-flow heat exchanger into a fifth liquid part and a sixth liquidpart; passing the fifth liquid part from the output of the thirdmulti-flow heat exchanger to the second drum separator; feeding thesixth liquid part from the output of the third multi-flow heat exchangerto a fourth drum separator to obtain a vaporized liquid and a nonvaporized liquid; forwarding the vaporized liquid obtained from thefourth drum separator to a second stage of the third compressor;combining and forwarding the non-vaporized liquid from the fourth drumseparator and the non-vaporized liquid from the second drum separator tothe demethanizer.
 4. The method according to claim 1, wherein the inputgas is a light gas containing a low amount of hydrocarbons that areconverted into a natural gas liquid.
 5. The method according to claim 1,wherein the natural gas liquid has a lower volatility degree as comparedto that of the residue gas.
 6. A method of external refrigeration forseparating a residue gas from an input gas to recover a natural liquidgas through a propane-refrigeration process comprising steps of: feedingan input gas into an internal refrigeration system; condensing the inputgas in a first multi-flow heat exchanger; separating an output of thefirst multi-flow heat exchanger into a fluid stream and a gas streamusing a first drum separator; dividing the separated gas stream obtainedfrom the first drum separator into a first gas part and a second gaspart; dividing the separated fluid stream obtained from the first drumseparator into a first fluid part and a second fluid part; passing thefirst fluid part obtained from the first drum separator to a middlesection of a demethanizer column through a Joule-Thomson expansion valvefor further fractionating; passing the second fluid part obtained fromthe first drum separator to a top rectifying section in the demethanizercolumn through a second multi-flow heat exchanger and a first expansionvalve to form a residue gas; passing the first gas part to a workexpander device; passing a gas stream output of the work expander deviceto an area below a top rectifying section in the demethanizer column asan expander discharge; passing the second gas part through the secondmulti-flow heat exchanger and a second expansion valve to the toprectifying section in the demethanizer column to form a reflux liquid;passing the residue gas from the top rectifying section of thedemethanizer column through the second multi-flow heat exchanger and thefirst multi-flow heat exchanger to a first compressor for furthercompression, and wherein the first compressor compresses the residue gasusing a work extracted from the expander device; passing a compressedresidue gas output from the first compressor to a second compressor forfurther compressing the residue gas; passing the further compressedresidue gas through a first air cooler; forwarding an output of thefirst cooler as a gas refrigerant; stripping off volatile componentsfrom a liquid collected at a bottom portion of the demethanizer columnby circulating liquids from four liquid draw trays provided in thebottom portion of the demethanizer column through the first multi-flowheat exchanger and the second multi-flow heat exchanger and collectingback the liquids at the demethanizer column; recovering the liquids froma bottom portion of the demethanizer column; and providing an externalrefrigeration process using liquid propane.
 7. The method according toclaim 6, wherein a step of stripping off volatile components from theliquid collected at the bottom portion of the demethanizer columncomprises: drawing a liquid from a first liquid tray and passing throughthe second multi-flow heat exchanger before being collected back at thedemethanizer; drawing a liquid from a second liquid tray and passingthrough the second multi-flow heat exchanger before being collected backat the demethanizer, wherein a direction of flow of the liquid from thefirst liquid tray through the second multi-flow heat exchanger and adirection of flow of the liquid from the second liquid tray through thesecond multi-flow heat exchanger are mutually opposite; drawing a liquidfrom a third liquid tray and passing through the first multi-flow heatexchanger before being collected back at the demethanizer; drawing aliquid from a fourth liquid tray and passing through the firstmulti-flow heat exchanger before being collected back at thedemethanizer, wherein a direction of flow of the liquid from the thirdliquid tray through the first multi-flow heat exchanger and a directionof flow of the liquid from the fourth liquid tray through the firstmulti-flow heat exchanger are mutually opposite.
 8. The method accordingto claim 6, wherein a step of providing external refrigeration usingpropane comprising steps of: feeding a compressed liquid propane from athird compressor into a second air cooler; forwarding an output of thesecond air cooler to a second drum separator through a third expansionvalve; passing a liquid propane output from the second drum separatorthrough a fourth expansion valve; forwarding a vapor gas output ofliquid propane from the second drum separator to the third stage of athird compressor passing an output liquid propane passed from the seconddrum separator through the fourth expansion valve; dividing an outputliquid propane passed through the fourth expansion valve into a firstliquid propane part and a liquid propane part; feeding the first liquidpropane part output from the fourth expansion valve to a firstmulti-flow heat exchanger for providing a required refrigeration forcooling the feed gas; forwarding the first liquid propane part outputfrom the first multi-flow heat exchanger to a third drum separator;feeding the second liquid propane part output from the fourth expansionvalve to the third drum separator directly through a sixth expansionvalve; forwarding a vapor gas output of the third drum separator to asecond stage of the third compressor; feeding a non vaporized liquidoutput from the third drum separator to a fourth drum separator afterpassing the non vaporized liquid output from the third drum separatorthrough a seventh expansion valve, the second multi-flow heat exchangerand the first multi-flow heat exchanger; passing a vaporized output fromthe fourth drum separator to a first stage of the third compressor; andforwarding a liquid propane output from the fourth drum separator as anexternal refrigerant.
 9. A system for separating a residue gas from aninput gas to recover a natural liquid gas comprising: a selfrefrigeration unit comprising an open-closed cycle of refrigeration; andan external refrigeration unit; wherein the self-refrigeration unitcomprising an open-closed cycle of refrigeration provides a requiredrefrigeration load and wherein the external refrigeration unit providesa refrigeration load corresponding to variations in a feed compositionpercentage.